Purpose:
Retinitis pigmentosa (RP) is an inherited retinal dystrophy caused by a progressive and irreversible loss of photoreceptors. RP prevalence is approximately 1/4000 worldwide and it may be transmitted in all inheritance patterns. To date, 36 genes have been associated with autosomal recessive retinitis pigmentosa (arRP). Due to arRP’s phenotypic and genetic heterogeneity, its molecular diagnosis is highly complex and time-consuming. The aim of this work was to identify the causative mutations in a Mexican inbred family with arRP.

Methods:
A total of 4 affected siblings were clinically evaluated. Patients underwent full ophthalmologic examination including fundus examination, ERG, FAG, and OCT. Molecular analysis was performed in genomic DNA from affected subjects and a number of non-affected relatives. A genome wide linkage analysis was performed on DNA from 3 patients by means of an Affymetrix 250K single-nucleotide polymorphism microarray. Exome sequencing of one affected family member was done using the TrueSeq exome enrichment kit, captured using Nimblegen SeqCap EZ V2.0 probes kit, and paired-end sequenced on an Illumina Hiseq 2000 platform at Ambry Genetics (Aliso Viejo, CA, USA). Dideoxy sequencing was used for candidate gene variant confirmation.

Results:
All affected patients exhibited dense intraretinal pigment migration, severe retinal pigment epithelium atrophy, and arteriolar attenuation, with a severe atrophic pigmentary maculopathy. A large region of homozygosity was observed at chromosome 18q. The TUBB6 gene was sequenced as a candidate gene but no mutations were demonstrated. Exome sequencing analysis disclosed two pathogenic mutations in the RDH12 gene: c.295C>A (p.L99I) and c.446T>C (p.L149P). Sanger sequencing demonstrated segregation of both RDH12 mutations in all affected subjects.

Conclusions:
While homozygosity mapping is an effective tool for identification of the underlying causative gene in inbred families, compound heterozygosity may occur even within the same consanguineous family. Whole exome sequencing is a powerful and cost-effective tool for molecular diagnostics in families with inherited retinal dystrophies.